Air Handling Units (AHUs), Filters and Fans: HVAC Monitoring with Differential Pressure Switches, Alarms and Maintenance

Differential pressure is one of the most useful parameters for assessing the operating conditions of an air distribution system. Although it does not provide a complete picture of system performance, it helps identify variations that may indicate developing operational problems.
In air filters, an increase in pressure drop may indicate progressive particulate accumulation. In fans, a differential pressure above or below the expected operating range may indicate an abnormal operating condition. In air handling units, differential pressure monitoring can support the supervision of filter sections, fans and specific sections of the air distribution system.
The key point is that differential pressure should not be interpreted as an isolated measurement. It must be associated with a specific function: identifying whether a filter is clean or loaded, verifying fan operation, detecting when an alarm threshold has been exceeded, scheduling maintenance or providing a permissive signal to the control system.

Filter monitoring is one of the most common applications for differential pressure switches.
Over time, an air filter develops increasing resistance to airflow as particles accumulate. This results in a progressively higher pressure drop between the upstream and downstream sides of the filter.
A differential pressure switch can be set to change its contact state when the pressure drop exceeds a predefined threshold. This provides maintenance personnel with a straightforward signal: the pressure drop has reached the specified limit and the filter requires inspection.
However, this does not mean that the pressure switch can independently diagnose the cause of the problem.
It cannot automatically distinguish between a clogged filter, incorrect installation, variations in airflow or another anomaly within the air distribution system. What it does provide is a useful indication that the differential pressure has reached a level requiring attention.
For this reason, differential pressure switches are particularly effective when integrated into a clearly defined maintenance procedure involving an established threshold, an understandable alarm, a verifiable maintenance intervention and a record of the action performed.

Another common application involves fan operation monitoring.
In many HVAC installations, confirming that an electrical start command has been issued is not sufficient. It is also important to verify that the air handling section exhibits conditions consistent with the expected fan operation.
A differential pressure switch can support this function by providing an operating permissive signal or triggering an alarm when the differential pressure fails to reach the configured threshold.
Here again, it is essential to understand the component's limitations.
A differential pressure switch does not directly measure the fan's actual airflow rate. Instead, it indicates whether the measured differential pressure is consistent with the specified switching threshold.
This distinction is important during system design.
If accurate airflow measurement is required, different measuring instruments and control strategies must be considered. However, when the objective is to confirm that a predefined pressure threshold has been reached, a differential pressure switch can provide a simple, robust and easily interpretable solution.

One of the most common mistakes in HVAC system design is confusing a differential pressure switch with a differential pressure transmitter.
A differential pressure switch is essentially a threshold-operated switching device.
When the differential pressure rises above or falls below the configured setpoint, the electrical contact changes state.
It is therefore suitable for managing discrete events, such as:
- Filter contamination alarms
- Fan operation permissive signals
- Control interlocks
- Local alarm indications
- Digital inputs to an electrical control panel
A differential pressure transmitter, on the other hand, provides a continuous output signal.
It is the more appropriate solution when the application requires pressure trends, historical records, BMS monitoring, analysis of operating conditions over time or proportional control.
In a properly designed HVAC system, these two technologies are not necessarily alternatives.
They can perform different functions within the same control architecture.
The transmitter provides information about how pressure changes over time, while the pressure switch identifies when a specific pressure value becomes an actionable event.
Before selecting a component, it is therefore important to define the required function.
Is a switching threshold sufficient, or is continuous measurement necessary? Is the objective to generate an alarm or monitor a trend? Is an operating status confirmation required, or does the system need continuous regulation?

The effectiveness of differential pressure monitoring depends heavily on selecting the correct switching threshold.
A threshold set too low may generate unnecessary notifications or false alarms. Conversely, a threshold set too high may delay maintenance intervention, reducing the effectiveness of the monitoring function.
For air filters, the switching threshold should be determined by considering the maximum permissible pressure drop, design airflow rate, filter manufacturer's recommendations and the maintenance strategy adopted for the installation.
For fan applications, the actual operating characteristics of the air handling section and the intended function of the permissive signal must be evaluated.
Hysteresis and switching tolerances are also important parameters.
A pressure switch operating too close to the normal working pressure may cause unstable switching and repeated alarm activation.
Conversely, an excessively wide switching differential may delay the detection of conditions requiring attention.
The practical principle is straightforward: the selected threshold must represent a meaningful operating condition for the personnel responsible for managing the HVAC system, rather than simply a theoretical value chosen from a product catalogue.

Selecting a differential pressure switch for HVAC applications should begin with an assessment of several essential technical parameters:
- Adjustable pressure range
- Switching differential (hysteresis)
- Electrical contact configuration
- Maximum permissible pressure
- Ingress protection (IP) rating
- Mounting orientation
- Compatibility with air as the operating medium
- Ease of calibration and setpoint adjustment
The installation environment must also be considered.
A device installed on an air handling unit or inside a technical room should be easy to inspect, accessible for maintenance and compatible with the electrical control panel or building management system.
For low-pressure applications, air filters and ventilation systems, selecting a suitable range of differential pressure switches for HVAC monitoring is a logical approach.
However, component selection should always be based on the required operating function and technical specifications rather than on product availability alone.
An overview of the available solutions can be found on the Comer website.

An effective alarm is one that leads to a clearly defined action.
If a differential pressure switch indicates excessive filter loading, maintenance personnel should know exactly what needs to be inspected.
If it signals an abnormal fan operating condition, technicians must be able to distinguish between a genuine malfunction, an incorrect switching threshold, a connection issue or a change in operating conditions.
For this reason, a differential pressure switch should never be considered an isolated component.
It must be integrated into a complete monitoring and maintenance process involving:
- Correct installation
- Documented switching thresholds
- Clearly identified alarm conditions
- Periodic functional checks
- Recorded maintenance activities
This approach helps prevent two opposite problems: ignoring useful warning signals or generating repeated false alarms that eventually become disregarded or disabled.

The first mistake is using a pressure switch as if it were a pressure transmitter.
If continuous measurements, historical records or pressure trends are required, a threshold-operated switching contact is not sufficient.
The second mistake is applying the same threshold logic to filters, fans and technical rooms.
These applications have different operating characteristics and require different monitoring criteria.
The third mistake is configuring pressure thresholds without considering hysteresis, switching tolerances and actual system behaviour.
In low-pressure HVAC applications, these parameters can significantly affect monitoring reliability.
The fourth mistake is using generic alarm messages.
A notification such as "HVAC fault" provides less useful information than an alarm identifying the specific filter, fan or air handling section involved.
The fifth mistake is assuming that installing a differential pressure switch automatically guarantees effective maintenance.
The component indicates a specific condition. The effectiveness of the monitoring system depends on how that condition is interpreted and managed.

In HVAC systems, a differential pressure switch is far more than a secondary accessory.
It provides an essential link between the physical operating conditions of the installation and the decisions required for effective system management.
In air handling units, filters and fans, it can convert an increase in pressure drop or an abnormal differential pressure condition into a useful signal for maintenance, alarm activation or fan operation verification.
However, its effectiveness depends on proper system design.
A correctly selected and adjusted differential pressure switch, integrated into a clearly defined maintenance strategy, helps make HVAC operating conditions easier to monitor and interpret.
Conversely, a pressure switch installed without a clearly defined function may produce signals that are ambiguous or of limited practical value.
The correct selection process should therefore begin with the intended function:
What should happen when the differential pressure exceeds or fails to reach the specified threshold?
Only after answering this question does it make sense to select the appropriate device, adjustment range and connection logic for integration into the HVAC control system.